1. Field of the Invention
The invention relates to semiconductor devices useful, for example, for converting electromagnetic radiation into electrical energy, as in solar cells, or for converting electrical energy into electromagnetic radiation as in light emitting devices.
2. Description of the Prior Art
Devices made by the interface of a wide bandgap window region and a narrower bandgap active region are being explored for solar cell and light emitting applications. Such devices formed by the interface of a transparent highly conductive layer (conductivity greater than 10.sup.2 .OMEGA..sup.-1 cm.sup.-1) and a direct gap semiconductor layer have inherent advantages in applications such as solar power conversion. The electrical contacts made to either the window or active region of a heterojunction device often have high resistances which limit obtainable solar power conversion efficiency. The use of a window region with high conductivity eliminates problems often associated with making a low resistance, nonrectifying front contact. A second important attribute of highly conductive materials is that they are often deposited by methods adaptable to commercial production processes. For example, indium tin oxide has been deposited in a variety of ways. (For a few illustrations see Fraser, D. B., Proceedings of IEEE, 61, 1013-1018, (1973) sputtering; Groth, R. and Kaver, E., Phillips Technical Review, 26, 105, (1965) pyrolysis; and Kawe, J. et al, Thin Solid Films, 29, 155-163 (1975) chemical vapor deposition.)
Despite the advantages of using a highly conducting window layer, there are often counter balancing disadvantages. For example, the interface of a highly conducting layer with a less conductive semiconductor usually forms a Schottky-type barrier. Such barrier devices often have lower efficiencies than those obtained with typical heterojunction semiconductor devices. The difficulty of fabricating a thin film polycrystalline device, i.e., a device with a polycrystalline active semiconductor region, having a highly conducting window region is also a significant problem. The window must be thin enough to transmit a substantial percentage, e.g., at least 50% of the incident solar energy. For highly conductive materials such as metals, this requirement often dictates a window layer thickness less than 150 Angstroms. However, when such thin layers are deposited on the relatively irregular surface of the thin polycrystalline active semiconductor layer, a continuous coating is usually not obtainable. These coating discontinuities make preparation of useful polycrystalline thin film devices with highly conductive window regions impractical.
Therefore a device having a highly conducting window region and a single crystal active region with an acceptable power conversion efficiency is a desirable entity. Such a device is particularly advantageous if the polycrystalline embodiment, i.e., the device with a highly conducting window region and a polycrystalline active semiconductor region, is operative.